Preserve blank lines before a closing fmt: on comment

CHANGES.md

@@ -17,6 +17,10 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Preserve blank lines that come immediately before a `# fmt: on` comment. They are
+  inside the opted-out region, but they live in the prefix of the `# fmt: on` comment
+  rather than in the verbatim block, so they were being capped like any ordinary blank
+  run (#5300)
 - Stop treating a t-string in docstring position as a docstring (for example
   `t"  spam  "` as the first statement of a module, class or function). t-strings
   evaluate to `Template`, never `str`, so stripping and reindenting one changed the

src/black/lines.py

@@ -5,6 +5,7 @@
 from typing import NamedTuple, Optional, TypeVar, Union, cast
 
 from black.brackets import COMMA_PRIORITY, DOT_PRIORITY, BracketTracker
+from black.comments import FMT_ON, contains_fmt_directive
 from black.mode import Mode, Preview
 from black.nodes import (
     BRACKETS,
@@ -1009,7 +1010,16 @@ def _maybe_empty_lines(self, current_line: Line) -> tuple[int, int]:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
             before = first_leaf.prefix.count("\n")
-            before = min(before, max_allowed)
+            # The blank lines that terminate a `# fmt: off` region live in the
+            # prefix of the `# fmt: on` comment, not in the verbatim block, so
+            # capping them here would edit formatting that was opted out of.
+            if not (
+                first_leaf.type == STANDALONE_COMMENT
+                and contains_fmt_directive(first_leaf.value, FMT_ON)
+                and self.previous_line is not None
+                and self.previous_line.is_fmt_pass_converted()
+            ):
+                before = min(before, max_allowed)
             first_leaf.prefix = ""
         else:
             before = 0

tests/data/cases/fmtonoff9.py

@@ -0,0 +1,40 @@
+# Regression test for https://github.com/psf/black/issues/2877.
+# Blank lines that terminate a `# fmt: off` region are inside the region, so
+# they must be preserved rather than collapsed to the usual maximum.
+x = 1
+# fmt: off
+a = 1
+
+
+
+# fmt: on
+y = 2
+
+
+def f():
+    x = 1
+    # fmt: off
+    a = 1
+
+
+
+
+    # fmt: on
+    y = 2
+
+
+# yapf: disable
+b = 1
+
+
+
+# yapf: enable
+z = 2
+
+
+# fmt:off
+c = 1
+
+
+# fmt:on
+w = 2